Hi,
I was recently looking up a tow package weight rating for a 2008 F-150 truck.
The weight rating was reduced for several reasons, including the 18" or 20" rims. Looking at the 2014 the tow guide did not mention de-rating due to rim diameter. Prior years did de-rate the Raptor package, so you probably need to carefully read the towing guide for the current information.
this is from the 2008 towing guide.
"Quote
(1) Reduce GCWR and Maximum Trailer Weight by 500 lbs. on models with 18" or 20" wheels. (2) Optional Ford Harley-Davidson™ Package and Lariat Limited.
(3) Requires Heavy Duty Payload Package. (4) Available with XLT Trim and LT245/75R17E A/T BSW tires only. (5) Available with XL Trim and LT245/70R17E A/T BSW tires only.
End Quote"
You can look up all the Ford towing guides here.
http://www.fleet.ford.com/towing-guides/The interesting part is you can now look up the new 2015 trucks and replacement full size Transit van. Full details of the 2015 towing will not be published until September 2014 though.
Fred.